Plasma arc gouging gains ground in the field of metal gouging

Often the same plasma torch is used for both arc gouging and cutting. It’s the consumables that are different. Images: Hypertherm Inc.

The popularity of plasma arc gouging has grown recently as a potential replacement for carbon arc gouging. The main drivers for this technology shift are the environmental benefits and improved safety.

Plasma cutting systems with gouging capability have been available on the market for more than 20 years. Initially, popular applications included removing tack welds and damaged welds, removing unwanted metal from parts like riser pads and castings, as well as chasing out cracks in damaged pieces of metal for repair.

Applications have extended to hardface removal and backgouging. Many companies are looking to automate these plasma arc gouging applications for the productivity gains and safety improvements, but also to be able to get consistent gouge shapes that are conducive to automated welding.

Here we examine the appeal of the process and the key factors in making your plasma arc gouging process a success.

Plasma and Carbon Gouging Compared

Air carbon arc gouging uses a carbon graphite electrode, electrode holder, and high-amperage power source (often an older welding power source repurposed for the job). High-pressure air is used to blow the molten metal away in the process.

The use of a carbon electrode or rod, by definition, means carbon will inevitably embed and fuse with the base metal. Before any welds are applied on the part, the carbon first must be removed. The grinding required to prep the surface tends to be extensive, but without it, you risk porosity and weakening the weld.

Plasma gouging, conversely, involves only a plasma arc so there is no risk of carbon deposits. If you are performing manual gouging with a hand-held plasma torch, it still may be necessary to grind out any ripples created in the process, but that grinding should be relatively light by comparison.

One beauty of air carbon arc gouging, and why it remains popular with some, is the predictability. You can choose a rod diameter and shape, round or flat, and predict the width of the groove being roughly 1/16 in. wider than the rod. The depth of the groove is most determined by amperage and rod feed speed. Depending on your choice of rod diameter and shape, you just adjust the amperage of your machine as recommended by the rod manufacturer.

With plasma, several additional variables determine the shape of the groove. Similar to air carbon arc, amperage output and speed in the plasma process affect the groove. The higher the amperage, the wider and deeper the groove will be, as well as the rate at which the unwanted metal or weld can be removed.

The torch angle and distance from the torch to the workpiece also have an effect on the groove shape and metal removal rates. For instance, increasing the standoff of the torch will increase groove width and decrease depth as it allows the arc to spread out, whereas decreasing the standoff of the torch will decrease width and increase depth. Increasing the angle of the torch (more vertical) will decrease width and increase depth; conversely, decreasing the angle of the torch (less vertical) will increase width and decrease depth.

The torch angle and distance from the torch to the workpiece have an effect on the groove shape and metal removal rates.

The angle of the plasma torch to the workpiece will also impact the shape of the gouge. When the torch is moved away from the plate or weld, the arc is less focused, allowing the width of the arc to spread wider, creating a U-shaped groove rather than a V shape. If you’re doing a backgouge and the groove shape is too deep, it can be very difficult to get a grinder in the groove to prep the surface. It can also be challenging to get a full-penetration weld in a V-shaped groove because of the steep included angles and the shallow bottom radius. The ideal groove has a depth and height that are about the same and a nice round bottom radius. The risk of porosity is greatly reduced with this groove shape.

Another variable with plasma gouging is the gas you choose. Compressed or bottled air offers the lowest cost of operation, but ultimately the decision will depend on the type of base material you’re working with. Other plasma gases might be more advisable; for instance, an argon/hydrogen gas mixture is useful for creating clean, bright gouges on aluminum and stainless steel.

Plasma Arc Gouging Consumables

Often the same plasma torch is used for both gouging and cutting. It’s the consumables that are different. There are specific consumables for gouging, so you’ll need to choose the right ones for the job. For example, when compared to the plasma cutting nozzle, the gouging nozzle has a bigger orifice so the arc can spread out to create a shallow groove. You’re not trying to cut through the plate.

Another consumable used specifically for the gouging process is the gouging shield. The gouging shield design redirects some of the gas flow from around the arc to use it for blowing the molten metal away from the gouging process.

Plasma Gouging Techniques

Hand Gouging. Hand gouging with a plasma arc requires a fair amount of technique. When you consider the angle, travel speed, and standoff requirements, it’s very much like putting down a quality weld bead. If you have a lot of weld or unwanted metal to remove, the technique is generally to do a single, straight pass. If you don’t remove everything in that first pass, then it’s common to make additional passes or to use a weaving technique, moving the torch from left to right as you move forward on the pass, sweeping molten material out of your path. This process is what can create the ripple effect mentioned in the first section of this article. Light grinding may be required until you get more comfortable with the process.

Mechanized Plasma Gouging. Manufacturers like Hypertherm are busy developing ways to make mechanized plasma gouging more effective and easier to set up and operate for repeatable results. A torch height controller that maintains a consistent torch-to-work distance on out-of-round or uneven plate, for instance, ensures that the groove profile is consistent from start to finish. When combined with tractors that have been designed for plasma cutting and gouging, which can travel at the optimal speeds and provide torch oscillation, larger groove profiles can be produced with high quality.

PPE for Safe Gouging

Safety First. With any plasma process, use the right personal protective equipment for the job. Check the lens shade on your helmet to confirm it is a shade 12. Since the plasma arc is more visible when gouging, a higher lens shade is required.

Protect Your Ears. Carbon arc gouging is extremely noisy, which is another reason shops are looking for alternatives. The combination of flowing high-pressure gas and repeated arc strikes makes it very loud. Plasma gouging is loud, but less so than carbon arc. Check your hearing protection and make sure it meets requirements by local regulations and that your daily exposure to the noise doesn’t exceed acceptable levels.

Protect Your Skin. Whether you are gouging using a hand-held torch or mechanized gouging, wearing the proper clothing and skin protection is important. Unlike plasma cutting where most of the plasma arc is below the metal, the gouging arc is fully visible as it is above the metal. As a result, exposure to ultraviolet light and sparks can be hazardous. The key is to choose the right fire-retardant clothing and to ensure all exposed skin is covered.

Increasing the standoff of the torch will increase groove width and decrease depth as it allows the arc to spread out, whereas decreasing the standoff of the torch will decrease width and increase depth.

Clear the Air. Fume extraction is critical to ensuring your safety. Draw fumes away from you with fume extraction equipment that is suitable for gouging applications, and wear a respirator. Be sure to follow all local regulations when it comes to fume extraction and mitigation methods as well as other forms of safety precautions mentioned above.

There are a variety of both thermal and mechanical gouging options available to fabricators. Thermal methods such as air carbon arc and plasma are indeed the most common, and time will tell if plasma arc gouging becomes the more desired method of removing unwanted metal and welds.

As regulations and customer requirements change, so must manufacturers in developing better hand and mechanized plasma gouging systems that not only meet customer production and design requirements in terms of groove shape, edge quality, and removal rates, but also improve workplace safety for operators as well as those who work in areas where gouging is performed.

Editor Robert Colman can be reached at rcolman@canadianfabweld.com. Contributor Harry Mellott is project manager with Hypertherm Inc., 21 Great Hollow Rd., Hanover, N.H. 03766, 603-643-3441, www.hypertherm.com.

Arc Gouging Tips for Best Results

• Various gouging nozzle and shield options are available depending on the system you are using and how aggressive or precise your gouge work needs to be. Be sure to check the specifications before starting your job.
• Some plasma machines also have a gouge mode. When this mode is on, the gas pressure is lowered. Gouging doesn’t require as much pressure as cutting.
• When gouging, you want to blow the molten metal out of the gouge, but you don’t want to blow metal too far outside the work area. It can interfere with other processes in the shop. This is considered another advantage of plasma compared to carbon arc. Carbon arc requires a lot of air to be effective, and it blows sparks farther. Carbon arc creates a more globular metal spark as well, which means that when those sparks land, they have a greater mass and tend to burn longer.

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